In optical transmission systems of the related art, binary intensity modulation (IM) is widely used. Also referred to as on-off keying, binary IM is a method wherein the logical “1” and “0” values of a signal to be transmitted are respectively associated with large and small optical intensities. This method has the merit of allowing for a simply-constructed transmitter and receiver, and for this reason is widely used at present.
Meanwhile, in recent years optical duobinary modulation is gathering attention as another method. Optical duobinary modulation is similar to binary IM, in that large and small optical intensities are associated with logical “1” and “0” values. However, in this method, two optical phases, 0° and 180°, are used for the high-intensity signal corresponding to the logical value “1”, and the baseband bandwidth is decreased.
In optical duobinary modulation, the optical signal intensity waveform is the same as that of binary IM, and for this reason can be received using a conventional optical receiver. In addition, since the baseband bandwidth is decreased, the spectral width is reduced and dispersion resistance is increased. For this reason, it becomes possible to increase the transmission distance or increase the number of channels in a WDM system. Given the above, recent attention has been focusing on optical duobinary modulation, and the development of optical transmitters implementing optical duobinary modulation is actively moving forward.
An exemplary method for modulating an optical duobinary signal and an exemplary configuration of an optical transmitter that generates an optical duobinary signal are disclosed in Japan Patent No. 3657983 and Japan Patent No. 3306573. As disclosed in the above literature, a method is known wherein a lithium niobate (LN), Mach-Zehnder (MZ) optical modulator is driven at an amplitude equal to or slightly less than 2Vpi (i.e., double the half-wavelength voltage), a bias voltage being applied to the modulator at the point where the optical output power is at a minimum (i.e., the null point of the characteristic transmission curve).
Although it is necessary in optical duobinary modulation to apply a bias voltage at the point where the optical output power is at a minimum (i.e., the null point of the characteristic transmission curve), the null point of an LN-MZ modulator tends to drift. For this reason, means for automatically controlling the bias voltage are required when constructing the optical transmitter.
As disclosed in Japan Patent No. 3723358, one method of realizing the above is known wherein a low-frequency signal is superimposed on an input electrical signal input into an LN-MZ modulator. This low-frequency component is detected from the output light of the LN-MZ modulator, and the bias voltage is controlled on the basis of this detected value.
In the biasing method as part of a system for generating an optical duobinary signal disclosed in Japan Patent No. 3723358, a modulator referred to as a dual-drive LN-MZ modulator is used to achieve zero-chirp modulation by push-pull driving two input terminals respectively provided in association with the two paths of a split waveguide.
However, when constructing an optical duobinary transmitter that uses a dual-drive LN-MZ modulator, it is necessary to provide two independent modulator drive circuits. Moreover, if the two independent drive circuit timings are not accurately matched and amplitude balance is not maintained, then accurate driving of the modulator cannot be assured.
Meanwhile, in recent years a modulator referred to as a single-drive LN-MZ modulator has been developed and put into practice, wherein push-pull driving is achieved inside the modulator using a single input. W. Kaiser et al. have proposed an optical duobinary transmitter constructed using a single-drive LN-MZ modulator (W. Kaiser et al., “Reduced Complexity Optical Duobinary 10-Gb/s Transmitter Setup Resulting in an Increased Transmission Distance,” IEEE Photonics Technology Letters, vol. 13, pp. 884-886, August 2001).
When using a single-drive LN-MZ modulator, construction of the transmitter is simplified, as only one modulator drive circuit need be provided. In addition, although LN-MZ modulators are constructed to be highly symmetric, it is not necessary to match drive circuit timings or amplitude balance in the construction of the transmitter, and thus the construction and adjustment thereof is simple.
However, with an optical duobinary transmitter using a single-drive LN-MZ modulator, it is not possible to independently apply voltages to the two respective paths of a split waveguide. For this reason, the bias voltage cannot be controlled using conventional methods, such as the method disclosed in Japan Patent No. 3723358, for example.